In the x-ray imaging art it is common practice to house sheets of film within cassettes which serve to support and mechanically protect the film as well as to shield it from ambient light. Such cassettes are generally square or rectangular in shape and come in a variety of sizes.
After being loaded with a sheet of unexposed film, the cassette itself must be loaded into the x-ray imaging machine and centered at a desired location prior to being exposed to radiation which has been directed through a portion of the body of a person or some other object to be imaged. Following exposure, the cassette must be removed from the imaging machine so that the exposed film can be removed from the cassette and developed. The evolution of the prior art has brought a variety of cassette handling systems for facilitating these operations.
As exemplified by U.S. Pat. Nos. 3,826,922 to Ingles and 3,829,698 to Goetz, one general approach has been to provide the x-ray imaging machine with a slidable drawer or Bucky tray for supporting the cassette. The interior of the tray is provided with a cassette positioning device for properly centering cassettes of different sizes. Some trays have been provided with a matrix of properly spaced holes into which pins can be inserted adjacent the edges of the cassette to hold it on center. Those are slow to use when cassette sizes must be changed and loose pins can become lost or jam the equipment.
Other positioning devices used inside trays, such as shown in the aforementioned Ingles patent, include two opposed pairs of cassette clamps movably mounted in guide slots and connected to a linkage of rigid members. Such linkages are constructed so that each clamp in a pair can be moved congruently toward and away from one another to effect bi-directional centering of random sizes of cassettes. These proved easier to use and therefore more efficient than the pin-matrix designs but suffered from a disadvantage common to all designs using trays. Namely, the tray itself required considerable clearance adjacent the x-ray machine to permit the tray to be fully open. To overcome this drawback, so-called "trayless" cassette transport systems have been developed.
An example of such a trayless device is described in commonly assigned U.S. Pat. No. 4,485,733 to Dieterlen et al. which is expressly incorporated herein by reference in its entirety. That device provides for the loading and unloading of a cassette into an x-ray apparatus through a window. Upon partial insertion of a cassette into the window, the cassette is engaged near its leading edge between opposing jaws of a gripper assembly which is slidably mounted on a movable carriage. Then, a pair of channels which move congruently toward one another with respect to a given axis are moved into engagement with the sides of the cassette to center the cassette about that axis. The gripper then withdraws the cassette inwardly by a distance related to the size of the cassette to center it about a second axis. The entire carriage, including the gripper, is then translated to move the cassette to an exposure position. Following exposure, the carriage is translated in the opposite direction to position the cassette in alignment with the window. To eject the cassette, the gripper is then moved toward its home position where it both stops and releases the cassette. The means by which it does so can be better understood with particular reference to FIG. 4 of the Dieterlen et al. '733 patent as described below.
The gripper assembly (40) of the '733 patent includes a cassette supporting plate (42) which carries a bearing block (48) upon which a spring-loaded upper jaw (52) is pivotably mounted. The upper jaw (52) is linked to an opposed lower jaw (64) mounted on the other side of plate (42). The lower jaw carries a roller which moves either upward through an opening in plate (42) to engage a detent on the cassette or downward through the opening to release a cassette. A spring (70) normally biases jaws (52) and (64) together to grip a cassette with a force that is limited by the strength of spring (72). However, when the gripper assembly is driven toward the window to its home location, a cam follower (73) that is connected to the lower jaw (64) by means of a follower (74) engages a cam ramp (76). This engagement overcomes spring (72) and opens jaws (52) and (64) so that they release the cassette.
A drawback of the above design is that the gripper cannot release the cassette until the gripper is positioned at its home position which lies a fixed distance from the window. If the home position is so located with respect to the window that a readily graspable portion of a small cassette projects unsupported from the window when the small cassette is ejected, an even larger portion of a large cassette will lie unsupported when that large cassette is ejected. This can result in the large cassette being precariously balanced upon ejection.
Thus, there is a need for an x-ray cassette transport apparatus which ejects a cassette by releasing it and arresting the ejection movement of the gripper at a desired ejection point whose distance from the window is determined in accordance with the size of the cassette.
It is also observed that in the gripper assembly described in the Dieterlen '773 patent, the force with which a cassette is gripped depends on the stiffness of the spring (70). Thus, for secure gripping, a fairly stiff spring must be used. However, the spring can grow weaker over time. Also, no matter how stiff the spring, there is no guarantee that the cassette will not be subjected to a withdrawal force greater than that which the spring can effectively counteract. These possibilities increase the risk of a cassette coming loose from the gripper and becoming improperly positioned or jammed inside the x-ray machine.
It can also be appreciated that in the gripper of the Dieterlen '733 patent, the motor which drives the gripper home must be powerful enough to overcome the spring. The requirement of having a stiff spring is contrary to the design objective of providing a less powerful and therefore less expensive motor. Accordingly, there has also been a need for an x-ray film cassette transport which can be driven with a motor of modest size, but which grips a cassette securely. In particular, there has been a need for a transport apparatus having a self-locking cassette gripper designed so that the gripping force exerted on a cassette increases in response to and at a greater rate than a cassette withdrawal force.